This invention relates generally to a mine roof support unit, and more particularly to such a unit having a supporting shield being pivotally supported on a floor skid at the backfilling end of the structure by a pair of rocker arms forming a lemniscate guide. A hydraulic actuator is pivotally interconnected between the skid and one of the rocker arms, and the other arm is longitudinally adjustable with its opposite ends being pivotally connected to said and to the shield and being so constructed as to permit the roof cap and a roof cap joint between the cap and the shield to follow a lemniscate path during settling of the roof while the actuator is made rigid, and to follow a circular path during a lowering of the cap toward the skid upon actuation of the hydraulic actuator.
A mine roof support structure generally of the type described herein is known to include a so-called circular arc shield wherein the supporting or breakage shield is connected to the floor skid at a pivotal joint so that the roof cap joint between the roof cap and the shield moves in a circular arc from one thickness of deposit (coal seam) to another. Thus, during a settling of the roof supported by such a structure, a force directed toward the backfilling end of the structure is introduced, and, as a result, cracks appear in the roof of the coal seam in a disadvantageous manner or cracks which are already present are further opened.
On the other hand, the known mine support structure having a so-called lemniscate (a figure-eight shaped curve) shield operates differently. It is possible with such structure, by an appropriate selection and support of the rigid rocker arms disposed between the floor skid and the breakage shield, to adjust the roof cap joint so as to follow a path inclined obliquely to the coal face. During settling of the roof supported by such a structure, the roof cap introduces a force directed toward the coal face into the roof of the seam which thereby prevents the formation of cracks in the roof of the seam or closes those cracks which are already present. However, when the seam converges, i.e., during a lowering of the roof cap and roof cap joint toward the skid, the roof cap joint moves along the same path inclined obliquely to the coal face, although it moves in a direction of the sill of the seam and, as a result, a force directed toward the backfilling end of the structure is introduced into the roof of the seam.